Rakuten, which runs the largest e-commerce site in Japan, is expected to be the lead investor in the much-contested next round of funding for Silicon Valley’s hottest start-up, Pinterest.

The funding is expected to be announced tomorrow morning.

(Update: Rakuten confirmed the deal in a press release, which is below.)

The Tokyo-based Internet giant will invest upwards of $50 million in a $100 million round that values the social bookmarking phenom at $1.5 billion.

There might be other individual investors in the new round, but those were still to be determined tonight by Pinterest co-founder and CEO Ben Silbermann.

(Correction: We previously reported it was a $120 million round, but sources said it is $100 million. It’s possible more could still be added, as we heard conflicting accounts.)

While the latest round of funding for Pinterest has been the most hotly sought of late in tech circles, one source said Silbermann was looking for a global strategic investor and had talked to several large Asian companies.

Said one source on why he settled on Rakuten: “He just really liked them.”

The current investors in Pinterest, which is a social collection site where users can “pin” their interests via a handsome graphical interface, include Andreessen Horowitz, Bessemer Venture Partners and FirstMark Capital, as well as several well-known angel investors. That group had valued the company at $200 million last October, and are joining the new round pro rata.

Founded in 2008, Pinterest had previously raised a little under $40 million in funding.

It’s interesting that the latest round was not led by an institutional investor, since everyone and their mother wanted in on the deal. Sources said no new venture capital firms were included in the round and that is the way Pinterest’s quirky leadership wanted it.

Sources said Silbermann has been concerned with Pinterest’s global growth as well as fending off international clones, and was looking for a partner with which the start-up could work closely.

“Ben did not want any more VCs,” said one source. “He wanted an investor that moved the company forward.”

It’s a good choice of partners in that regard. Rakuten is one of the largest e-commerce companies in the world, with a flagship site Rakuten Ichiba. It was founded in 1997 and had revenues of $4.7 billion in 2011. Its CEO is Hiroshi Mikitani, whose nickname is Mickey.

One the richest men in Japan, Mikitani is one of the best known entrepreneurs there; he has been described as “Richard Branson meets Jeff Bezos.”

Earlier today, The Next Web reported that the funding was coming this week and said the company was looking at international partners, but it did not name Rakuten.

Rakuten Leads Investment In Pinterest

Global social commerce pioneer takes stake in online sharing service

TOKYO, May 17, 2012 — Rakuten, one of the world’s largest online marketplaces, today announced that it is leading a $100M investment in Pinterest, with participation from existing investors Andreessen Horowitz, Bessemer Venture Partners, and FirstMark Capital, as well as a number of angel investors.

The funding will allow Pinterest to continue improving its service and expanding its community globally. The investment also marks the start of a strategic partnership between Rakuten and Pinterest to help expand in Japan and into Rakuten’s 17 other global markets.

Hiroshi Mikitani, CEO of Rakuten said: “While some may see e-commerce as a straightforward vending machine-like experience, we believe it is a living process where both retailers and consumers can communicate, discover, and curate to make the experience more entertaining. We see tremendous synergies between Pinterest’s vision and Rakuten’s model for e-commerce. Rakuten looks forward to introducing Pinterest to the Japanese market as well as other markets around the world.”

Ben Silbermann, co-founder and CEO of Pinterest, said: “Our goal is to help people discover things they love, by connecting people through their shared interests. Bringing Rakuten on board gives us an amazing opportunity to move a step closer to this goal.”

Rakuten ranks among the top 10 internet companies in the world. Among its numerous online properties, its flagship B2B2C (business-to-business-to-consumer) model e-commerce site Rakuten Ichiba is the largest e-commerce site in Japan and among the world’s largest by sales. Its global presence has been reinforced through the acquisitions of leading online marketplaces Buy.com (US), Priceminister (France), Ikeda (now Rakuten Brasil), Tradoria (now Rakuten Deutschland) and Play.com (UK), and investments in Ozon.ru and AHA Life. Whereas other marketplaces may compete directly with sellers, Rakuten’s model seeks to empower merchants to deliver Omotenashi, a Japanese high service mindset, which helps sellers create lasting relationships with customers.

And here’s a video of Kara pontificating away on the subject for WSJ.com’s “Digits” show:

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